Alternatively, if you would rather edit the registry by hand... (This assumes familiarity with RegEdit.exe.)

Go to HKEY_LOCAL_MACHINE\Software\Classes\.htm

Ensure there is a Content Type value with Type = REG_SZ (String), and Data = text/html

Go to HKEY_CURRENT_USER\Software\Classes\.htm

If there is no .htm key at that path, or there is one but it has no Content Type value, you are done and can stop.

Otherwise, ensure it is set to text/html as with the HKEY_LOCAL_MACHINE one.

Multiple Users:

If your have multiple user accounts on your machine, you may need to apply the HKEY_CURRENT_USER part of the fix to each account separately.

There is no need to reboot or restart Opus after applying the fix. The help documents should display correctly as soon as you open a new help window (F1). (Close any existing help windows first, of course.)